The present invention relates generally to a fluid flow sensor for determining the velocity and/or the amount of fluid flowing therethrough, which is applicable to, for example, the fuel induction passage of an internal combustion engine to measure the amount of fuel supplied to the engine. More specifically, the invention relates to an improvement to a fluid flow sensor in use with a movable ball member which is moved by the fluid flow at a speed proportional to the fluid velocity.
Various fluid flow sensors for measuring fluid velocity and/or fluid amount flowing therethrough have recently been developed. Among the various types of fluid flow sensors, a fluid flow sensor employing a movable ball disposed within a fluid passage and moving at a speed corresponding to the fluid velocity is known. Such a fluid flow sensor is available on the market for use, for example, engines. engine mountable to an automotive vehicle and so on.
In the general construction of such conventional fluid flow sensors, the ball member is disposed within an annular fluid passage and moves in a circumferential direction. The fluid introduced into the fluid passage flows along the annular passage and moves the ball member therewith. On both sides of a portion of the passage, a photo-emitting member and a photo-sensitive member are provided in lateral alignment with respect to the circumferential axis of the passage. The ball member interrupts the light emitted from the photo-emitting member. These interruptions are counted up, and based on the counted value, the fluid velocity and/or amount is determined.
In such a conventional construction of the fluid flow sensor, the motion of the ball member is substantially affected by friction between the ball member and the internal periphery of the annular fluid passage. Due to this friction, the speed of the ball member is apt to be lower than the fluid velocity. As a result, this kind of fluid flow sensor does not respond so accurately to a change of fluid velocity, particularly in the range where the flow energy of the fluid is relatively small. When such a fluid flow sensor is added to the fuel induction passage of the internal combustion engine, the sensor has to respond to a wide range of fuel velocities, for example, while the engine operating condition is varied from idling position to full load position. However, within the range in which engine operating condition has a small variation, the sensor cannot accurately follow the variation of fuel velocity and/or the amount of fuel flow. Therefore, the accuracy and reliability of the measurement are substantially lower than that required.